Recovery of waste gases in the production of sulphite pulp



Sept. 1l, 1928.

G. A. RICHTER RECOVERY 0F WASTE GASES IN THE PRODUCTION OF SULPHITE PULP Filed March 19, 1925 Patented Sept. ll, Y192%..

TES

lGrECfRG'rZE A. RICHTER, OF BERLIN," NEW HAMPSHIRE, .ASSIGNOR T0 BROWN COMPNYD F BERLIN', NEW MPSHIRE, A GORP()RATION' OF Y `ItIllCOVERS.' 0F WASTE GASESLIN THE PRO`DUCTION 0F UIILPEITE PUL'.

rApplication fue@ March-1a, 1925. sen-ai No. 16,679.

This invention has for its object an improved method of recovering sulphur dioxide gas from the vapors and gases which are liberated with the discharge of the contents 5 of a sulphite digester, following the comple-y tion of the cooking operation by which wood chips are digested in an acid sulphite liqu`or, and the utilization of the recovered S2 gas in the production of fresh cooking liquor having a high percentage of free sulphur dioxide gas.

Tn the normal operation of cooking wood s chips for the production of sulphite pulp, it

is customary to coolr the chips in a closed dit gester under proper conditions of heatand pressure until thefcellulose fibers are lfree from the ligneous-or encrusting components of the wood, and then to discharge the contents ot the digester into a blow pit wherein the pulp or cellulose fibers are separated from the waste cookingliquor. With the liberation of the liquid content of the digester which has previously been subjected to high pressure and temperature, the stream issuing from the blow pipe is approximately at or slightly below the boiling point so that much of the contained water vaporizes and has mixed therewith a relatively large percentage of ree'SOZ.v

Tn accordance with the present invention,

the mixed vapors and gases from the blow pit are caused to pass through a condensing tower provided with inert interstitial material and into direct contact with water which is sprayed or delivered to the top of l,the tower s'o as to pass the gas as, the liquid travels downwardly through the interstitial material. It has heretofore been customary to utilize for this purpose fresh water at .seasonable temperatures The water -condenses the .vapor and cools the gas, which as' it leaves 40% to 50% by volume of SO2', which it is the purpose o'i the present'process to recover apparatus, it may be stated thatcrdinarily in-the production of an acid sulphite liquor,

sulphur or sulphur-bearing material is burned the condenser towen comprises approximately thereinto.

may-Abe employed in carrying out the process tion are caused to pass through one or more lime towers in serieskthrough which water is passed for the formation of what may be termed a raw acid liquor. This raw acid liquor ordinarily contains about 1.0% combined SO2 and 2.5% :tree S02. The rawacid liquor thus produced must be strengthened by the addition of free S02 tobring it to a point at which it ycontains approximately 5% to 6% free SZ and thus be ready for use in the digesters for the liberation ot fiber and the production ot sulphite pulp.

Referring to the accompanying drawing- 1 indicates a -sulphite digester of the usual type in which wpod chips (spruce tor examu ple) are cooked 1n an acid sulphite liquor,- usually calcium bisulphite.

chips and the liquidcontent of the digester together with such free gas as may be therein are discharged through a blow pipe 2 into ay n Upon the com? pletion ot the cooking operation, the cooked `for the production of SO2, and the burner gases?? so-called, resulting from this combus wise closed and is provided with a laterally projecting portion 6 provided with ports 7 on its lower side throughwhich the' vapors and mixed gases are emitted. This laterally projecting portion of the stack extends into the lower end of a tower indicated at 8 whichV is provided with spiralbrick or-other inert4 Vinterstitial material as indicated at 9. A

stream of liquid to be described is delivered through apipe 10 to the top of the tower and is sprayed downwardly therein so as to pass downwardly through the inert interstltial material in n 'iultitudinous streams in counter-" current flow to the gases and vapors which "rise upwardly through the tower. The condenser tower is subjected to a artial vacuum to ensure the flow of vaponan -gas upwardly At 11 and 12 are indicated two towers each of which is provided with-a mass i of inert interstitial material such as spiral brick, the upper portion of tower 12 being" connected by pipe 13 with a vacuum pump indicated conventionally at 14. A plpe or conduit 15 leading from the top of the condenser, tower 8 is connected to the lower end of'tower 11, and similarly a pipe or conduit 16 extends from the' top of tower 11 `to the lower part of tower 12. The partial vacuum created by .and in passing the pump 14 extends back into the condenser tower 8 as will be readily appreciated. These two towers 11 and 12 constitute means for recovering the SO2 which passes from the blow pit upwardly through the condenser 8.

In accordance with the present invention, water at seasonable temperatures is delivered to the top of the second recovery tower 12,

downwardly through the interstitial material absorbs such SO2 from the blow pit mixed gases as were not absorbed in tower 11. The weakly aeidulated water which is delivered from the bottom of tower 12 is conducted by a pipe 17 to a large tank indicated at 18 located in convenient proximity to the condenser 8. It is this acidulated water in what I may call the weak acid liquor tank 18 which is utilized for condensing the vapors and cooling the gases in the condenser. A pump 19 withdraws the weak acid liquor from the tank 18 and forces it through pipe 10 to the top of the condenser tower 8.

Omitting for the moment further reference to the tower 11, attention is directed to the two lime towers which are indicated at 20 and 21, and which are utilized in producing what I term the raw acid li uor. Burner gases, i. e. the roducts of com ustion resulting from the burning of sulphur or sulphur-bearing material, after being properly cooled are delivered by` a pipe 22 to a fan or blower 23 by which they are delivered to the bottom of tower 21. This tower, like tower 20, is partially filled with limestone, dolomite or other calcium-bearing material. Water at seasonable temperatures is delivered to the top of lime tower 20 through a pipe 24, and in passing downwardly therethrough meets a current of gas sup lied to the bottom of tower 20 by a pipe 25 eading from the top of tower 21. The liqior from the bottomof tower 20 is pumped y a pump 26 through pipe 27 to the to of tower 21, where, in passing through t e limestone or dolomite, it comes in contact therewith and also with the fresh burner gases to the lower end of tower 21.

' The acid liquor from the bottom of lime tower 21 is conducted by a pipe 28 to what I have termed for convenience of, phraseology or designation the intermediate acid liquor tank 29.' It is Lfrom this tank that the raw acid liquor is caused to vcirculate through the blow pit gas recovery tower 11 to increase its strength in free SO2.' To this end the acid liquor is withdrawn from intermediate acid liquor tank 29 by a pump 30 andy forced through pipe 31 to the top of tower'11, and, after being passed through the interstitial material therein, the liquor is conducted back to the tank 29 by a sealed pipe 32. Although the acid liquor in the intermediate acid liquor tank 29 has its proportion offree SO2 increased, its strength must be further increased so that it may be utilized to the greatest advantage as a cooking liquor. For the further fortification or strengthening of the intermediate acid liquor, a substantially pure SO2 gas is required, and this is supplied by relief gases which are relieved from time to time from the digesters during the cookinvr operation. This relief gas is supplie( through a pipe or conduit indicated at 33 and it is delivered to the lower end of what for convenience I term the final acidifyinff tower 81. This tower is partially filled with inert interstitial material, such for example as spiral bricks. The intermediate acid liquor from the tank 29 is pumped to the top of this tower by pump 35 and pipe 36. The final or finished acid liquor is delivered from the bottom of tower 34 to a tank indicated at 37 which for convenience of designation I term the finished acid liquor tank.

Thus it will be seen from the foregoing description that the finished acid liquor is acidified by burner gas, by gas recovered from the blow pit, and by cold relief gas.

Assuming that the digester 1 is of fairly large size, about 10,000 to 16,000 gallons of water are required for the condenser 8, deplending upon the temperature of the conenser water, to condense the vapors and to cool the gases which `have been delivered from the digester upon the completion of the cook. The supply of condensing water form weak acid liquor tank 18 to the top of the condenser is so regulated ordinarily that this water leaves the lower end of the condenser at a temperature ranging from 190o to 200 F. and is delivered by a trapped pipe 38 into the blow pit so as to impinge upon the stream being delivered into the blow pit through the blow pipe 2. As the condensing water and the condensate are heated by the hot stream of chips and liquor emerging from the blow pipe, they are freed from SO2 which was absorbed thereby, and this SO2 passes upwardly with the 'other vapors and gases through the stack 5 back into the condenser. Such SO2 as is not absorbed by the condensing water passes through the pipe 15 into the tower 11, and it comprises 50% to 60% of SO2 by volume of the mixed gases, affording therefore a strong acid gas which may be utilized in strengthening the intermediate acid liquor which was delivered to the tank 29 from the lime towers 20, 21. The cold blow pit gases, which have been unabsorbed in the tower 11, now pass to tower 12 where they meet a stream of fresh water at atmospheric temperatures and are absorbed thereby. The gas leaving tower 11 contains ordinarily 15% to 18% of SO2 by volume, and' Y `Lcsaoae the risiny steam and vapors so that its con` tained 8%), is driven therefrom and passes through pipe to tower 11 with the other SO2 gas delivered from the blow pit. The

' the gases leaving the condenser 8 are increased and the gas which was absorbed in the weak recovery tower 12 is liberated therefrom without the use of extraneous heat additional to ,that supplied by the'digester 'contents. Furthermore the increased volume and concentration of cool blow pit gases allow a correspondingly high vfortification or increase in strength of the raw acid liquor ldelivered from the limetowers to the intermediate acid liquor tank 29.

The sulphur burner gases contain ordi` nai-ily about 16%.,SO2, andthe raw acid sulphite liquor leaving tower 21 contains about 2.5% free SO2 and 1.0% combined SO2 as it is delivered to the tank 29. By circulating this liquor through the `first recovery tower` 11', where it is brought into Contact with a gas containing to 60% SO2,.it is converted to what I have termed an intermediate acid liquor containing 3.0% to 3.2% free SO2 and 1.0% combined SO2. Then lthis intermediate acid liquor is brought into Contact with a 'gas which is substantially pure SO2, namely 'the cold relief gas which comprises 90% to 100% SO2 by passing it through tower 34 and is then converted to` a final acid liquor containing 5% to 6% free SO2 and 1% combined SO2. ln this way the sulphite liquor is subjected to increasingly stronger'or more concentrated gas, and .by a series of these steps "is converted into its hnal form. Substantiallyl no gas'is wasted and all but a trace ot the blow pit gases are recovered.

The advantages'gained bythe operation hereinbefore described may be increased or exaggerated by diverting a portion of the sulphur-burner gases through tower 12 (with the gases from recovery-tower 11) for the i purpose of increasing the acid strength of the weak liquor delivered therefrom to a point where it contains up 'to 2% SO2. 7ifhis, consequently, increases the strength in SO2 of the g gases leaving the blow pit condenser 8 suiiicientlyto increase the .acidY strength of the intermediate liquor leaving the recovery/,tower il ltuinay be reiterated that, bv the method herein described. there is a saving of heat in recovering SO2 from the blow pit (in that SU? absorbed by the cooling water is liber- Llated by the heat of the digester contents). an increased S02 concentration in the chilled from e carena. e@ scare 2% ne@ so..A

gases leaving the blow pit condenser is secured, substantially all of the SO2 from the blow Vpitis recovered except such minute' amounts as are absorbed by and wasted with th-e waste liquor from the blow pit, and an acid sulphite cooking liquor having from 5%' to 6% free S()2 is-produced. at relatively low expense and with a marked economy in its production.

What I claim is 1. A process of recovering SO2` from hot- ,absorbed thereby, and supplying suchacidulated water to the first-mentioned mass of interstitial material for the condensation and cooling of blow-pit vapors and gases and liberation of the SO2 contained in such acidu- I lated water.

2; A process of recovering SO, fromblowpit vapors and gases and utilizing the same in the production4 of bisulphite cooking liquor, which comprises conducting` such vapors and gases through inert interstitial material, wherein the vapors are condensed, passing the chilled gases through a mass of inert interstitial material into contact with a bisulphite liquor to increase the acid strength of such liquor, passing the resulting tail gases through a mass of interstitial material 'in contact with water for the acidulation of such water by the remaining SO2, and passing the acidulated water through the rst- .mentioned material to condense and cooLthe said blow-pit gases and vapors passing therethrough and toliberate the SO2 ly absorbed in such-water.

3. A process of recovering SO2 from blowgas previousducing .a' raw acid' sulphilte liquor by con. f

ducting water and. sulphur burner gases in contact with calcium-bearing material; conducting blow-pit vapors and gases through an inert surface material and condensing such vapors and chilling the gases;fcirculating said raw acid liquor through inert interstitial material and conducting such chilled blowpit gases through said last-mentioned. mate- Arial in contact with such raw acid liquor to increase the acid strength of such liquid; passing the unabsorb'ed tail gases in contact with water through inert interstitial material for the absorption of the remaining SO2 and the ac idulation of such water; and parseing such acidulated water through the firstinentioned inert material in contact with the blow-pit vapors-and gases and in countercurrent {iowthereto to condense such vapors, liberating the SO2 from such acidulated wachill such gases, and liberate the gas preter; causing said raw acid liquor to absorb viously absorbed by such water. a portion of the S()2 from the chilled blow-pit 4. A process of recovering SO2 from blowgases; then bringing such acid liquor to ultipit vapors and gases and utilizing the same mate acid strength by contact with substaninthe production of bisulphite cooking liquor tially pure SO2 gas; and effecting the acidurich in free SO2, which comprises producing lation of said Water by the absorption therein a raw acid sulphite liquor, condensing and of SO2 remaining in said chilled blow-pit chilling the said blow-pit vapors and gases gases. bv SO2 acidulated Water and simultaneously 6. A process of producing a sulphite cookliberating the S()2 from such -acidulated waing liquor having a high proportion of free ter; causing said raw acid'liquor to absorb SO2; which comprises first producing a raw a portion of the SOL2 from the chilled blowacid sulphite liquor, circulating such liquor pit gases; and effecting' the acidulation of through inert surface material in contact Said water by the absorption therein of SO2 with chilled blow-pit gases to increase the remaining in said chilled blow pit gases. proportion of free SO2 therein; and then 5. A process of recovering SO2 from blow- :assing such liquor through inert material pit vapors and gases and utilizing the same in contact with digester, relief gases to proiu the production of bisulphite cooking liquor duce a finished acid cooking liquor having a rich in free SO2, which comprises producing high proportion of SO2. a raw acid sulphite liquor, condensing and In testimony whereof I have affixed my chilling the said blow-pit vapors and gases signature. by SO2 acidulated Water and simultaneously GEORGE A. RICHTER. 

